Owl Canyon. Rainbow Basin is a geological formation in the Calico Peaks range in the Mojave Desert. Majority of fossil beds are found within the sedimentary rocks of the Barstow Formation; include many animals not found in California today, including camels, horses, mastodons, and flamingos. National Natural Landmark (Est. 1966) and is in the Bureau of Land Management managed Rainbow Basin Natural Area. San Bernardino County, California.